FRIDAY, Aug. 31 (HealthDay News) -- Women older than 75 who have a heart rhythm disorder called atrial fibrillation are 20 percent more likely to have a stroke than men in the same age group with atrial fibrillation, according to a large new study.

However, researchers said the findings suggest that being a woman should not be included as an independent risk factor for stroke or blood clots, either in guidelines for treatment or risk assessment of patients with atrial fibrillation, because the difference doesn't hold for women younger than 75.

The study included more than 87,000 atrial fibrillation patients in Denmark. Of those patients, more than 51 percent were female.

After one year of follow-up, female patients younger than 75 did not have an increased risk of stroke, but those over age 75 had a 20 percent increased risk, the investigators found.

The study was presented Sunday at the European Society of Cardiology's annual meeting in Munich.

"Our findings could have an impact on current guidelines used in the treatment of atrial fibrillation; however, more research is needed to confirm our results," study author Anders Mikkelsen of Copenhagen University Hospital Gentofte, Hellerup, Denmark, said in a society news release.

Female sex has been suggested as a risk factor for stroke and blood clots in atrial fibrillation patients, but the 2010 European Society of Cardiology guidelines for management of atrial fibrillation consider being female a minor risk factor.

For women with atrial fibrillation, the guidelines recommend blood thinners for those who are younger than 65 and have one additional risk factor, and for those ages 65 to 74 with no additional risk factors.

Blood thinners can lower the risk of stroke and blood clots in atrial fibrillation patients, but they increase the risk of bleeding disorders and should therefore be given only to patients at high risk of stroke and blood clots.

Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.